Process for preparing an ion exchange membrane

Abstract

A novel process is disclosed in which a membrane of a fluorinated resin containing a pendant precursor ion exchange group represented by the formula: --SO.sub.2 X wherein X rerpesents a halogen atom, is contacted with a aqueous solution containing specific amounts of at least one alkali metal or alkaline earth metal hydroxide and at least one specific water soluble organic compound to thereby hydrolyze the precursor group while causing a specific membrane volume increase. In the process of the present invention , since the ion exchange membrane is swelled, while being hydrolyzed, to a desired degree, the rate of hydrolysis of precursor ion exchange groups is high, so that a final ion exchange membrane is free of wrinkles formed during the hydrolysis step of a conventional process and during the electrolytic operation, thereby enabling the final membrane to ensure advantageously low cell voltage and stable, prolonged electrolytic operation.

What is claimed is: 
     
       1. A process for preparing an ion exchange membrane comprising the steps of: (1) providing a membrane of a fluorinated resin containing a pendant precursor ion exchange group represented by the formula:   --SO.sub.2 X        wherein X represents a halogen atom,   said membrane having an equivalent weight of from 600 to 1600 and a thickness of from 100 to 200 μm, and   (2) contacting said membrane having said precursor group with an aqueous solution at a temperature of from 60° to 130° C. for a period of from 20 minutes to 24 hours,   said aqueous solution containing at least one alkali metal or alkaline earth metal hydroxide and at least one water soluble organic compound selected from the group consisting of dimethyl sulfoxide, methanol, ethanol and propanol,   the proportions of the alkali metal or alkaline earth metal hydroxide and the water soluble organic compound in said aqueous solution being, respectively, in the range of from 15 to 50% by weight and in the range of from 0.1 to 30% by weight, based on the weight of said aqueous solution,   thereby hydrolyzing said precursor group of formula --SO 2  X to a sulfonate group while causing a membrane volume increase of from 10 to 55%,   said membrane volume increase (ΔV) being defined by the following formula: ##EQU2##  wherein a represents the volume at 25° C. of the membrane having the precursor group before hydrolysis, and b represents the volume of the membrane after hydrolysis as measured in said aqueous solution at 25° C.   
     
     
       2. The process according to claim 1, wherein the proportion of the water soluble organic compound in said aqueous solution is in the range of from 1 to 25% by weight, based on the weight of said aqueous solution. 
     
     
       3. The process according to claim 1 or 2, wherein the water soluble organic compound is dimethyl sulfoxide. 
     
     
       4. The process according to claim 1, wherein said fluorinated resin provided in step (1) further contains a pendant precursor ion exchange group represented by the formula:   --COY     wherein Y represents a fluorine atom, a chlorine atom or --OR in which R represents an alkyl group having 1 to 3 carbon atoms,   said precursor group of formula --COY being hydrolyzed to a carboxylate group in step (2),   and wherein the ratio of precursor group --SO 2  X to the total of precursor groups --SO 2  X and --COY corresponds to at least 20% of the equivalent weight of the ion exchange membrane.   
     
     
       5. The process according to claim 4, wherein the ratio of precursor group --SO 2  X to the total of precursor groups --SO 2  X and --COY corresponds to at least 40% of the equivalent weight of the ion exchange membrane. 
     
     
       6. The process according to claim 1, wherein the fluorinated resin of membrane (1) is at least one copolymer obtained by reacting at least one fluorovinyl compound represented by the formula:   CF.sub.2 ═CF(--O--CF.sub.2 CFL).sub.m O--(CF.sub.2).sub.n SO.sub.2 X (1)     wherein L represents a fluorine atom or a perfluoro alkyl group having 1 to 3 carbon atoms; m is an integer of 0 to 3; n is an integer of 1 to 3; and X represents a halogen atom, with at least one fluorinated olefin represented by the formula:     CF.sub.2 ═CFZ                                          (2)     wherein Z represents a hydrogen atom, a chlorine atom, a fluorine atom or a perfluoro alkyl group having 1 to 3 carbon atoms.   
     
     
       7. The process according to claim 6, wherein m of formula (1) is an integer 1 or 2, and n of formula (1) is an integer of 2 or 3. 
     
     
       8. The process according to claim 6, wherein the fluorovinyl compound of formula (1) is represented by: ##STR2## 
     
     
       9. The process according to claim 6, wherein the fluorinated olefin of formula (2) is tetrafluoroethylene. 
     
     
       10. The process according to claim 1, wherein the membrane further comprises a reinforcing material encapsulated therein. 
     
     
       11. The process according to claim 10, wherein the reinforcing material is a chemically inert perhalocarbon polymer. 
     
     
       12. The process according to claim 11, wherein the chemically inert perhalocarbon polymer is a homopolymer of tetrafluoroethylene, a copolymer of tetrafluoroethylene and hexafluoroethylene, perfluorovinyl ether or a mixture thereof. 
     
     
       13. The process according to claim 10, wherein the reinforcing material is in the form of a woven fabric, a non-woven fabric or a porous article. 
     
     
       14. The process according to claim 10, wherein the reinforcing material is in the form of a stretched microporous polytetrafluoroethylene sheet. 
     
     
       15. The process according to claim 1, wherein the alkali metal or alkaline earth metal hydroxide of the aqueous solution is KOH, NaOH or a mixture thereof. 
     
     
       16. The process according to claim 1, wherein the alkali metal or alkaline earth metal hydroxide and the water soluble organic compound is in the range of 20 to 45% by weight. 
     
     
       17. The process according to claim 1, wherein the contacting temperature is from 60° to 100° C. 
     
     
       18. The process according to claim 1, wherein the contacting of step (2) is for a period of 30 minutes to 3 hours. 
     
     
       19. The process according to claim 1, wherein said membrane volume increase is from 15 to 44%.